Finasteride, honestly: how it works and what the side effect numbers really show
Finasteride has 25+ years of data behind it. Here is what it does, what it does not, and how to think about the side effects you have probably heard about online.
Key points
- Finasteride inhibits 5-alpha reductase, lowering DHT — the hormone driving male pattern hair loss.
- Roughly 90% of men maintain or regrow hair over 2+ years of use.
- Sexual side effects occur in a minority (single-digit percentages in trials); persistent symptoms are rare but real.
- It is a lifelong treatment — stopping returns you to your genetic baseline within ~12 months.
The mechanism
Androgenetic alopecia is driven largely by dihydrotestosterone (DHT), a more potent form of testosterone produced by the enzyme 5-alpha reductase. DHT gradually miniaturizes hair follicles on the scalp in genetically susceptible men.
Finasteride inhibits type II 5-alpha reductase, lowering DHT by roughly 70%. Less DHT means the miniaturization process slows — and in many men partially reverses.
What the long-term studies actually show
In the landmark 5-year study of finasteride 1 mg daily, 90% of men stopped losing hair and 65% showed measurable regrowth. Placebo groups continued to lose hair steadily over the same period. Benefits plateau around 12-24 months and are maintained as long as you keep taking it.
The counterfactual matters: "not losing more" is a real win for a progressive condition, even when it does not look dramatic in photos.
The side effect conversation, with numbers
In randomized trials, sexual side effects (lower libido, ED, reduced ejaculate volume) occurred in roughly 1-4% of men on finasteride versus ~1-2% on placebo. The absolute increase is small but real.
Post-finasteride syndrome — persistent symptoms after stopping — is rare and not well-characterized, but there is enough signal that clinicians take it seriously. If side effects appear, stopping almost always resolves them. The decision is ultimately a risk tolerance conversation.